mtd: spi-nor: Add steps for testing locking support

mtd: spi-nor: Add steps for testing locking support

As recently raised on the mailing list, it may be useful to propose a
list of steps to go through in order to prove the devices have been
described correctly, especially since all the block protection
information is not stored in any kind of table and is instead filled
manually by developers.

Use the debugfs output to ease the comparison between expectations and
reality.

[rdunlap@infradead.org: fix build warning]
Link: https://lore.kernel.org/linux-mtd/20260526172341.773398-1-rdunlap@infradead.org/
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>